1. Field
The present disclosure is generally directed to transmitting data and, more particularly, to techniques for transmitting data in a wireless communication system that employs quasi-orthogonal space-time block code.
2. Related Art
Antenna diversity, also known as spatial diversity, is a wireless diversity scheme that utilizes two or more antennas (at a transmitter and/or at a receiver) to improve the quality and reliability of a wireless link. When spatial diversity is achieved, a signal transmitted from each antenna experiences a different fading environment. As such, assuming a transmission from one antenna experiences a deep fade, it is likely that a transmission from another antenna will not experience a deep fade when transmission channels are not highly correlated.
Space-time code (STC) refers to an approach that may be employed to improve the reliability of data transmission in wireless communication systems. STC employs multiple transmit antennas and relies on transmitting redundant data copies to a receiver, via the multiple transmit antennas. STC is based on the premise that at least one of the redundant data copies survives a transmission path (between a transmitter and a receiver) in a state that allows reliable decoding. STCs are generally divided into two main types: space-time trellis codes (STTCs) and space-time block codes (STBCs). An STTC distributes a trellis code over multiple antennas and multiple time-slots and provides both coding gain and diversity gain. In contrast, STBCs act on a block of data and only provide diversity gain. In general, STBCs are less complex to implement than STTCs. STCs may be further subdivided according to whether a receiver has knowledge of channel impairments or channel statistics. In a coherent STC, a receiver has knowledge of channel impairments (e.g., through training or some form of estimation). In a non-coherent STC, a receiver has knowledge of channel statistics, but does not have knowledge of channel impairments. In a differential STC, neither channel impairments nor channel statistics are available.
STBCs exploit various received versions of data from respective data streams to improve the reliability of data transmissions. The fact that a transmitted signal traverses a potentially difficult environment with scattering, refraction, reflection, etc. (and may then be further corrupted by thermal noise at a receiver) means that some received copies of transmitted data are usually better than other received copies of transmitted data. In general, when channels are not highly correlated, redundancy improves the probability that one or more of the received copies of transmitted data may be decoded correctly. Typically, STCs attempt to combine all received copies of a transmitted signal in an optimal way to maximize information extraction from each of the received copies. An STBC is usually represented by a matrix, in which each row represents a time slot and each column represents a different antenna over time. Elements of the matrix correspond to symbols, where sij is the symbol to be transmitted in time slot ‘i’ from antenna ‘j’. In a system, there are generally ‘T’ time slots, NT transmit antennas, and NR receive antennas. A length of a code block usually corresponds to a length of ‘T’. A code rate of an STBC measures how many symbols per time slot are transmitted, on average, over the course of one codeword.
STBCs may be orthogonal or quasi-orthogonal. An orthogonal STBC is designed such that vectors representing any pair of columns taken from a coding matrix are orthogonal. In general, receivers may employ linear single-symbol decoding for orthogonal STBCs. A disadvantage of some orthogonal STBCs is that all but one of the orthogonal codes may sacrifice some proportion of their data rate. Alamouti code, which is an orthogonal STBC that was originally designed for a system having two transmit antennas, provides a full-rate code that transmits a symbol in each time slot for a system having two transmit antennas. Systems that employ Alamouti code with two transmit antennas can achieve full-diversity gain without sacrificing data rate, when complex symbols are employed. In general, there are no known orthogonal STBCs for a system having more than two transmit antennas that can achieve full-rate transmission. With orthogonal STBCs, maximum likelihood (ML) decoding can be performed at a receiver using linear processing. In contrast, quasi-orthogonal STBCs, which allow some inter-symbol interference, can generally achieve full-rate transmission and provide better error-rate performance under harsh transmission conditions, albeit while requiring more complex decoding procedures.
Several types of quasi-orthogonal space-time block codes (QSTBCs) with double-symbol decoding (i.e., pair-wise symbol decoding) have been proposed in an effort to achieve full-rate transmission for a system having more than two transmit antennas. As noted above, in contrast to orthogonal STBCs, QSTBCs have some inter-symbol interference. However, unless constellation rotation is employed, known QSTBCs only provide partial diversity gain. An STBC that implements a coordinate interleaved orthogonal design (STBC-CIOD) and a minimum decoding complexity QSTBC (MDC-QSTBC) have been proposed to achieve full-diversity gain and full-rate transmission with single-symbol decoding. However, the STBC-CIOD requires special procedures, such as constellation rotation and coordinate interleaving, at a transmitter. Moreover, the MDC-QSTBC requires constellation rotation and dispersive mapping of I and Q components of symbols to be transmitted. Furthermore, optimum rotation angles for STBC-CIOD and MDC-QSTBC are different for each modulation type (e.g., QPSK, 8-PSK, 16-QAM, etc.).